We used eddy covariance; gas-exchange chambers; radiocarbon analysis;
wood, moss, and soil inventories; and laboratory incubations to measur
e the carbon balance of a 120-year-old black spruce forest in Manitoba
, Canada. The site lost 0.3 +/- 0.5 metric ton of carbon per hectare p
er year (ton C ha(-1) year(-1)) from 1994 to 1997, with a gain of 0.6
+/- 0.2 ton C ha(-1) year(-1) in moss and wood offset by a loss of 0.8
+/- 0.5 ton C ha(-1) year(-1) from the soil. The soil remained frozen
most of the year, and the decomposition of organic matter in the soil
increased 10-fold upon thawing. The stability of the soil carbon pool
(similar to 150 tons C ha(-1)) appears sensitive to the depth and dur
ation of thaw, and climatic changes that promote thaw are likely to ca
use a net efflux of carbon dioxide from the site.